nitrogen oxides and their environmental role

Burn anything hot enough in air — in a car engine, a power station, a lightning strike — and some of the air's nitrogen and oxygen combine into nitrogen oxides, the family of small molecules collectively called NOx. They are colourless or brown gases, mostly invisible until they pile up, and they are among the most important and troublesome molecules in environmental chemistry.

Nitrogen forms a remarkable spread of oxides because it has so many accessible oxidation states. Nitrous oxide, N2O (laughing gas), is a fairly inert anaesthetic and a potent greenhouse gas. Nitric oxide, NO, is an odd-electron molecule (it has an unpaired electron, so it is a radical) that is, surprisingly, a vital signalling molecule in the human body, controlling blood-vessel dilation. NO is readily oxidised in air to nitrogen dioxide, NO2, a brown, choking, also-odd-electron gas that dimerises partly to colourless N2O4. There are higher oxides too (N2O3, N2O5). The two that drive pollution, NO and NO2, are usually lumped together as NOx because they interconvert so easily.

Environmentally, NOx is a central villain. In sunlight NO2 helps cook up photochemical smog and ground-level ozone, the brown haze over busy cities. NOx dissolving in atmospheric water makes nitric acid, a major component of acid rain that damages forests, lakes, and buildings. This is why car exhausts pass through a catalytic converter, whose job is partly to reduce NOx back to harmless N2. The honest framing: these are not exotic chemicals but the unavoidable by-product of burning fuel in ordinary nitrogen-rich air, which is exactly why controlling them is such a stubborn engineering and policy problem.